Dispersion experiments with tracer gas in Oslo, Norway D. T0nnesen Norwegian Institute for Air Research, P.O. Box 100, N-2007 Kjeller, Norway

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1 Dispersion experiments with tracer gas in Oslo, Norway D. T0nnesen Norwegian Institute for Air Research, P.O. Box 100, N-2007 Kjeller, Norway Abstract Dispersion experiments using tracer gas technique were carried out in Oslo during stable stratification and calm wind conditions. Three sets of data were collected, from a street canyon, from an open area with high houses, and across a valley with cold-air drainage flow. The tracer gas release was set up as a line-source in the two first cases, and as point-sources using two different tracers in the third case. Vertical concentration profiles were measured by balloon in the first case and with a drone in the two last cases. The horizontal and vertical tracer gas concentration profiles have been parameterized using a gaussian type dispersion model. The experiments show that the vertical dispersion caused by vehicle induced turbulence is considerable even during stable stratification, and that there can be considerable deviation in transport directions during drainage-flow situation depending on the location of the source within the valley. 1 Introduction Oslo, capital of Norway, is a city with approximately 1/2 million inhabitants. It is located in a basin at the north end of the Oslofjord and is surrounded by hills of heights varying from above 400 m to below 200 m, as shown on Figure 1.

2 208 Air Pollution Engineering and Management Figure 1: The topography of Oslo During the winter, local episodes with stable atmospheric stratification and calm wind conditions often occur when a high-pressure ridge is built up over the interior of the Scandinavian peninsula. During such episodes, the movement of air and the resulting advection of air pollution within the city is largely determined by drainage flows through the small valleys within the basin. On the street-scale, the turbulence generated by vehicles will have impact upon the dispersion and advection of air pollutants. Several field experiments using tracer gas technique have been undertaken by NILU to study both wintertime cold air drainage flows and dispersion and advection on the street-scale. These experiments have been motivated by the need to improve the modelling of air pollution advection and dispersion within the city. 2 Tracer gas technique The dual tracer gas system developed by NILU consists of a portable release system; syringe based samplers and portable gas chromatographs. The system has been designed to meet the following requirements: easy to modify, easy to handle, light weighted samplers with present timers rapid in field analyses The release system can be modified for different scale applications to give concentrations of from 10 to 105 ppt (parts per trillion). The two tracers that can be analysed simultaneously by the gas chromatographs in field are SFg and

3 Air Pollution Engineering and Management 209 The air samples are collected in inexpensive syringes instantaneously or integrated over a pre-set time period. Normally, the unit time period is 15 min. A number of battery operated samplers with a timer system for pre-setting of start/stop time have been developed by NILU. Also semi instantaneous sequential samplers and a continuous sampler have been built. A lightweighted semi instantaneous sequential sampler was also designed to be carried in a drone for sampling of concentrations above ground. 3 Experiments Cold air drainage flow For investigation of drainage flow through the valleys leading to Oslo central city from the east, a dual tracer gas experiment was performed. The experimental design is shown in Figure 2, with the release points for the two tracers and the selected lines where tracer gas concentrations were measured in points. Also shown in the figure is the cross-section where tracer gas concentration distributed above ground level was measured by a drone. The wind speed measured between the release points and the first set of sampling points was 2.5 m/s. Figure 2: Release points for tracer gas and measuring points for tracer gas experiment in Oslo, 23 January During the experiment 15-minutes average tracer gas concentrations were sampled in points, and average concentrations at levels of 60 m and 90 m above ground were measured over a 100 m cross section. The point measurements and the corresponding iso-concentration lines for the tracer gases are shown in Figure 3.

4 210 Air Pollution Engineering and Management Figure 3: Tracer gas concentration distributions in the field experiment at Oslo, 23 January The extension of the SFg tracer gas plume southwards is not completely determined by the mapped concentrations. However, the concentration distributions of the two tracer gases show that although the source areas for the tracer gases were separated by 1.2 km, the cross wind maximum concentration at a distance of 5 km from the source were separated by more than 2 km. This divergence in the advection field is caused by the topographical variations within the main valley. The average concentrations measured by drone at two levels, and the corresponding ground level average concentration as determined from the point samples are shown in Table 1. Tab lei: Average tracer gas concentrations for different vertical levels in the tracer gas experiment in Oslo, 23 January Level Ground 60 m 90m Concentrations (uq/m3) SFs CBrFs The crosswind concentration profiles of the CBri^ tracer were nearly completely covered within the sampling array. A "best-fit" gaussian distribution has been applied to them for each of three equidistant set of points. The results of this application are shown in Table 2.

5 Air Pollution Engineering and Management 211 Gaussian concentration distribution parameters derived from the tracer data. Distance (km) Maximum concentration (uq/m3) Observed Predicted Standard deviations (m) Gv GZ The SFg tracer was released near the lowest part of the valley, and probably confined mainly within the depression when transported past the first two lines of sampling points. After the second line of sampling points, the valley floor drops abruptly, and the vertical extension of the plume increases as reflected in the measurement of the average concentrations at the 60 m and the 90 m level. Street-scale dispersion Two sets of experiments are reported. The first set was carried out in 1987 within the center of Oslo city in a rectangular and uniform street pattern. The second set was carried out in 1993 at Majorstua in the north-central part of the city in an area with 8 to 10 storey houses with open space in between. Experiments in Oslo city, 1987 The experiments were focused on dispersion around Raadhusgata. All the streets in this area had one-way traffic at the time the experiments were performed. In four of the five experiments, dual tracer gas was applied; one tracer gas (SFg) was released along 600 m of the street, the other one (CBrFg) was released at roof level of a 20 m high building. The zones at points where tracer gas was released, are shown in Figure 4. Also indicated are the areas where tracer gas were collected. Figure 4: Site for tracer gas experiments in Oslo, January-March Zones for tracer gas release and general area of tracer gas sampling.

6 212 Air Pollution Engineering and Management The experiments were performed during situations with weak wind and stable stratification. The wind force was between 0.8 m/s and 1.8 m/s at roof level during all the experiments. Release rates for tracer gas and corresponding average concentrations in the street where tracer gas was released and the parallel streets are shown in Table 3. Table 3: Tracer gas experiments in Oslo, January-March Released rates for tracer gases and average concentrations in street and adjacent streets. Experiment Wind speed (m/s) Release rate (g/min) SF6 CBrF Average concentrations (pg/m^) 0 streets away 1 street away 2 street away SFfi CBrFs SFfi CBrFs SFfi CBrFs As shown by the CBrFg concentrations, there was considerable impact from the release at approximately 20 m above street level into the adjacent streets. As the test was performed during stable stratification, the vertical transport is probably caused by traffic-induced turbulence. The average ventilation rate of the street canyon has been calculated by the ratio between release rate and average concentration of the SF& tracer gas. The result is shown in Table 4. Average ventilation rate in m^/s in the street canyon per m of the street, expressed as ratio between release rate and concentration. Also shown is the wind speed at roof level. Experiment Variation rate Wind speed The table indicates that variation of the wind speed alone is not sufficient to explain the variation of the ventilation rate. Experiments at Majors tua, 1993 The experiments were carried out as dual tracer gas experiments, although the main purpose of the secondary tracer (CBrFg) was to serve only as an indicator to the direction of air movement through the sampling array. The primary tracer (SF&) was released from cars driving along the upwind side of the test area. Sampling of vertical profiles of tracer gas was attempted using a drone, but due to the tall buildings in the area, operations were difficult. Two sets of vertical profiles from 10 m to 200 m were collected, but these had to be sampled downwind from the main building area, approximately 350 m from the tracer gas release. These concentration profiles show a

7 Air Pollution Engineering and Management 213 rather uniform distribution up to 200 m, at a concentration level only slightly less than the measured ground level concentrations. This indicates surprisingly strong vertical dispersion in the area. Table 5 shows a summary of the test results, given as average downwind concentrations in streets parallel to the simulated line source at different distances. The wind speed during all the tests was below 1 m/s, In the first four tests, the tracer gas release rate was 1.27 g/min over a length of 540 m, and in the last four tests, the release rate was 1.9 g/min over a length of 550 m. Table 5: Average downwind concentrations (ug/m*) in parallel streets at different distances from the line source. Test No Distance (m) Assuming a uniform vertical distribution of tracer gas, and applying a simple flux consideration with wind speeds of 1 m/s, provides an average vertical extension of the tracer gas cloud as shown in Table 6. Table 6: Vertical extension of the tracer gas cloud assuming uniform distribution. Test No Distance (m) Although the computed heights show considerable variations from test to test at the same distances, this crude analysis supports the measurements of vertical extension carried out by the drone. An explanation for the significant vertical motion can be that local updrafts and downdrafts exist along the high and slim building in the test area.

8 214 Air Pollution Engineering and Management 4 Conclusions Dispersion Experiments performed within a length-scale of several kilometres show that under advection conditions dominated by cold-air valley flows, dispersion from ground level sources are highly dependent upon detailed terrain-features. When using a gridded windfield model during these conditions, the impact of such terrain features should be an integrated part of the model. Dispersion experiments with tracer gas technique provides a valuable tool for investigating small scale variations within the windfield during such conditions. Within built-up areas, considerable vertical dispersion take place even during stable stratification. When modelling dispersion of pollutants, the increased vertical dispersion within built-up areas should be taken into account. References Heggen, R. and Sivertsen, B. (1983) Tracer gas techniques at NILU. Lillestr0m (NILU TR 8/83). Gr0nskei, K.E. and F. Gram (1989) Exposure estimates in Oslo based on data for emission, dispersion and residential distribution. In: Man and his Ecosystem. Proceedings of the 8th World Clean Air Congress The Hague, The Netherlands. Volume 3, Gr0nskei, K.E. (1989) Description of vertical dispersion under influence of roughness elements. In: Air pollution modelling and its application VII. Edited by Han van Dop. (Plenum Publishing Corporation) Larssen, S., Gr0nskei, K.E., Gram, Frederic, Hagen, Leif Otto and Walker, Sam-Erik (1994) Verification of urban scale time-dependent dispersion model with subgrid elements, in Oslo, Norway. In: Air pollution Modeling and Its Application X, Plenum Press, New York 1994,